linux-kernel - [PATCH v2 2/2] mm/uffd: Sanity check write bit for uffd-wp protected ptes

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-Id: <20221110203132.1498183-3-peterx@redhat.com>
Date:   Thu, 10 Nov 2022 15:31:32 -0500
From:   Peter Xu <peterx@...hat.com>
To:     linux-kernel@...r.kernel.org, linux-mm@...ck.org
Cc:     peterx@...hat.com, Andrea Arcangeli <aarcange@...hat.com>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        Ives van Hoorne <ives@...esandbox.io>,
        Nadav Amit <nadav.amit@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>
Subject: [PATCH v2 2/2] mm/uffd: Sanity check write bit for uffd-wp protected ptes

Let's add one sanity check for CONFIG_DEBUG_VM on the write bit in whatever
chance we have when walking through the pgtables.  It can bring the error
earlier even before the app notices the data was corrupted on the snapshot.
Also it helps us to identify this is a wrong pgtable setup, so hopefully a
great information to have for debugging too.

Cc: Andrea Arcangeli <aarcange@...hat.com>
Signed-off-by: Peter Xu <peterx@...hat.com>
---
 arch/x86/include/asm/pgtable.h | 16 +++++++++++++++-
 1 file changed, 15 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h
index 5059799bebe3..64141acf70c8 100644
--- a/arch/x86/include/asm/pgtable.h
+++ b/arch/x86/include/asm/pgtable.h
@@ -291,7 +291,21 @@ static inline pte_t pte_clear_flags(pte_t pte, pteval_t clear)
 #ifdef CONFIG_HAVE_ARCH_USERFAULTFD_WP
 static inline int pte_uffd_wp(pte_t pte)
 {
-	return pte_flags(pte) & _PAGE_UFFD_WP;
+	bool wp = pte_flags(pte) & _PAGE_UFFD_WP;
+
+	/*
+	 * Having write bit for wr-protect-marked present ptes is fatal,
+	 * because it means the uffd-wp bit will be ignored and write will
+	 * just go through.
+	 *
+	 * Use any chance of pgtable walking to verify this (e.g., when
+	 * page swapped out or being migrated for all purposes). It means
+	 * something is already wrong.  Tell the admin even before the
+	 * process crashes. We also nail it with wrong pgtable setup.
+	 */
+	VM_WARN_ON_ONCE(wp && pte_write(pte));
+
+	return wp;
 }
 
 static inline pte_t pte_mkuffd_wp(pte_t pte)
-- 
2.37.3